Measuring characteristics of materials by using susceptive and conductive components of admittance

ABSTRACT

A system for measuring a condition of a material using an admittance probe or probes which are immersed in or placed in proximity to the material, in which both the susceptive and conductive components of admittance exhibited by the probes are utilized to provide measurements of the desired characteristic which are substantially free from errors due to variations in other characteristics of the materials.

United States Patent [191 Maltby MEASURING CHARACTERISTICS OF MATERIALSBY USING SUSCEPTIVE AND CONDUCTIVE COMPONENTS OF [451 July 17, 19733,566,259 2/l97l Wilson 324/61 R FOREIGN PATENTS OR APPLICATIONS 9l1,975 12/1962 Great Britain Y 324/65 R ADMlTTA-NCE 295,025 10/1969Australia i 324/65 R [75] Inventor: Frederick L. Maltby, Jenkintown,404,208 6/1970 Australia 324/65 R Pa. I

[73] Assignee: Drexelbrook Controls, Inc., Examiner-$311163 Kl'awclewiczI-lorsham,Pa. Attorney-Thomas M. Ferrill, Jr. and Allen V. 221 Filed:Aug. 20, 1971' 21 A LN 173538 1 pp 0 57 ABSTRACT 52 s CL 324 5 R, 73 304C, 324 R A System for measuring a condition of a material using 511 int.Cl G0lr 27/02 an admittance Probe or Probes which are immersed in 58Field of Search 324/65 R, 61 R; or Placed in Proximity to the'material,in which both I i 73/304 C the susceptive and conductive components ofadmitv i tance exhibited by the probes are utilized to provide [56]References Cited measurements of the desired characteristic which areUNITED STATES PATENTS substantially free fromerrors due to variations inother 2 008 046 7/1935 S 324/65 R characteristics of the materials.

, ne mg I 14 Claims, 14 Drawing Figures J3 v {7 f j] .1/ 1; 79 i/zm m au r; 0 j 20 uz- 42 l 1 4; 7 26 27 l/mmm m arc/114m 2:22; AMPl/f/IR W Ame{I an :2" 37 [J I a am 3 5 M0! 4 7,- 46 WA, 34) 12 i? 4/ 1 MEASURINGCHARACTERISTICS OF MATERIALS BY USING SUSCEPTIVE AND CONDUCTIVECOMPONENTS OF ADMITTANCE This invention relates to improvements inapparatusfor measuring various characteristics of materials.

For example, it is known to make measurments of the weight of granularmaterial by observing the susceptance exhibited by a probe immersed inthe material whose weight is to be measured. However, the accuracy ofsuch measurements has been subject to substantial errors because ofmoisture associated with the material to be measured which causes theprobe susceptance to vary not only as function of the dry weight of thematerial but also as a function of the moisture associated therewith.Also it has been observed that the probe exhibits a conductive componentwhich varies as a function of the moisture content of the material inwhich the probe is immersed. In some instances the conductance will varyas a function of the moisture content in accordance with substantiallythe same mathematical relationship as the susceptance, while in otherinstances it will vary in accordance with a substantially differentrelationship. I have determined that in either event it is possible tooperate on'the conductance in such a manner as to produce a quantitywhich, when subtracted from the susceptance, will yield a quantity whichis substantially independent of variations in moisture content and whichis representative substantially only of variations'in thedry weight ofthe material being measured. Further, since the quantity produced byoperating on the conductance is directly representative of variation inthe moisture content, it may be divided by the quantity representativeof the dry weight to yield a quantity representative of variations inthe percentage moisture content of the material being measured.

In accordance with the invention there are provided means for measuringboth he susceptive and the conductive components exhibited by a probeimmersed in material whose dry weight is to be measured. If the twoquantities vary in accordance with substantially the same mathematicalrelationship they are supplied directly to a differential amplifier orother suitable subtracting circuit, at theoutput of which is produced aquantity which will be directly representative of variations in dryweight of the material and independent of variation in moisture content.

If the two quantities derived from the measurements vary in accordancewith different mathematical relationships, a quantity representative ofthe ratio of conductance to susceptance is produced and is supplied to asuitable non-linear amplifier or function genrator adapted to modify itand produce a quantity varying in accordance with substantially the samemathematical relationship as the susceptive component. Then thesusceptive component and the quantity representative of the modifiedratio of the two components are supplied to the differential amplifier,at the output of which is produced a quantity representative ofvariations in dry weight.

In the first case, if desired, the modified quantity representative ofthe conductive component and the output of the differential amplifier,representative of dry weight, may be supplied to a conventional dividercircuit to produce at its output a quantity representative of percentagemoisture content.

In the second case a quantity representative of the ratio can besupplied to an amplifier to produce at its output a quantityrepresentative of the precent moisture content.

Another example of the applicability of the invention relates to themeasurement of conductivity between spaced points of a coating ofconductive material immediately after it has been applied to arelatively nonconductive substrate. Here it is not feasible to measurethe conductivity of the coating by probes directly contacting thecoating because it is still moist and sticky and would be disturbed bysuch probes. I have found that it is possible to make such measurementsusing spaced capacitive electrodes positioned in proximity to thesubstrate on the uncoated side thereof, by deriving signalsrepresentative of the susceptive and conductive components of theadmittance exhibited by such electrodes, and by operating on saidsignals to produce a signal which is representative of the ocnductivityof the coating and which is independent of variations in the thicknessof the substrate.

Yet another example of the applicability of the invention relates to themeasurement of level of conducting material in a vessel by an immersionprobe where the conducting material tends to produce a coating to alevel higher than the actual level of the liquid. In this case thecoating produces an increase in both the susceptance and the conductancemeasured between the probe and the grounded vessel. When the coating isrelatively long, and thus would produce appreciable error, the increasein conductance is equal to the increase in susceptance. In this case Ihave found that it is possible by subtracting the conductance from thesusceptance to produce a resulting quantity which will be representativeof the true level of the liquid.

While the foregoing represent typical examples of the manner in whichcertain characteristics of materials may be measured in accordance withthe present invention, it will be understood that numerous othercharacteristics may be measured utilizing the principles of theinvention in ways which will be apparent to those skilled in the art inview of the disclosure contained herein.

Accordingly it is an object of the invention to provide improvedapparatus for measuring various characteristics of materials.

More particularly, it is an object of the invention to provide apparatusfor measuring certain characteristics of materials while avoiding errorsin such measurements due to variations in other characteristics thereof.

Further it is an object of the invention to provide apparatus usingadmittance probes to measure certain characteristics of materials, inwhich both the susceptive and the conductive components of admittanceexhibited by the probes are utilized to provide measurements of saidmaterial which are substantially free from errors due to variations inother characteristics of the materials.

Another object ofthe invention is to provide apparatus for measuring thedry weight of granular material independently of variations in itsmoisture content and also for measuring the percentage moisture contentof such material.

Another object of the invention is to provide means for measuring theconductivity between spaced points of a conductive material applied tothe surface of a relatively non-conductive substrate.

Another object of the invention is to provide means for measuring a trueliquid level by an immersion probe without errors due to the conductingfilms on the probe.

The invention will be more fully understood from consideration of thefollowing detailed description thereof with reference to the drawings,in which:

FIG. 1 is a block diagram illustrating one form of the invention.

FIG. 1A is a block diagram illustrating a modification of the embodimentof FIG. 1.

FIGS. 2, 3 and 4 are graphs which will be referred to in explaining theoperation of the embodiments of FIG. 1- and 1A.

FIG. 5 is a block diagram illustrating a modification of the embodimentof FIG. 1.

FIGS. 5A and 5B are vector diagrams which will be referred to inexplaining the modification of FIG. 5.

FIG. 6 is a diagram illustrating a further modification of theembodiment of FIG. 1.

FIGS. 6A and 6B are equivalent circuit diagrams of the modification ofFIG. 6.

FIG. 7 is a block diagram illustrating a further modifi cation of theembodiemnt of FIG. 1.

FIG. 8 is a diagram illustrating a condition which may be encountered inoperation of the embodiment of FIG. 1, and

FIG. 8A is an equivalent circuit explanatory of the condition of FIG. 8.

Referring to FIG. 1, the output of an oscillator 10, designed to operateat a frequency of, for example, 200 khz., is supplied through acombining circuit 11 to the input ofa buffer amplifier 12 having a highgain e.g., well in excess of 1,000. The output of amplifier 12 is fedback to combiner 11 and combined with the output from oscillator toprovide substantially I00 percent negative feedback to provide a highlystable low impedance source of alternating high frequency signal at theoutput of amplifier 12, which is connected to the primary winding 13 oftransformer 14. The two halves 15 and 16 of the secondary winding oftransformer 14 respectively form two adjacent arms of a bridge circuit17. One of the remaining two arms of the bridge comprises the parallelcombination of resistor 18 and capacitor 19 representing respectivelythe conductive and susceptive components of admittance exhibited by aprobe 20 adapted for immersion in a material whose weight is to bemeasured, said probe being connected to the upper terminal of the upperhalf 15 of the transformer secondary winding through a coaxial cable 21having its outer conductor connected through connection 23 to point 19a.The other arm of the bridge comprises the parallel combination ofvariable resistor 24 and variable capacitor 25. The output of the bridgecircuit is developed across a capacitor 22 connected be tween thejunction 15a of the two halves l5 and 16 of the transformer secondarywinding and point 190. This output is supplied to the input of animpedance buffer amplifier 26, the output of which is supplied throughconnections 27 and 28 respectively to the input of RF. amplifiers 29 and30, both of which may have provisions for adjusting their respectivegains. The outputs of said amplifiers are supplied respectively to theinputs of phase sensitive detectors 31 and 32. Detector 31 also issupplied through connection 33 with an in-phase signal from the outputof buffer amplifier 12, and detector 32 is supplied through connection34 and phase shifter 35 with a quadrature-phased signal from the outputof buffer amplifier 12. Detector 31 operates to develop an outputproportional to the susceptive component of the admittance exhibited byprobe 20, and detector 32 operates to develop an output proportional tothe conductive component of the same admittance. The respective outputsof detectors 31 and 32 are filtered in low pass filters 36 and 37 toproduce substantially d-c voltages. The output of filter 37 is suppliedto the input of a variable voltage divider circuit which consists of apotentiometer 41 connected in series with a resistor 39 which has itsother end connected to the circuit common. The center arm of thepotentiometer 41 is the output of the variable voltage divider. Both theoutput of the variable voltage divider and the output of the low passfilter 36 are supplied to the input of a differential amplifier 42, theoutput of which is connectable by a switch 43 either to the input of anoutput d-c amplifier 44 or to the input of a divider 45. There is also aconnection 46 from the output of the variable voltage divider to theinput of divider 45. By means of switch 47 the output of divider 45 maybe connected to or disconnected from the input of amplifier 44 as willbe discussed later. The output of amplifier 44 may be observed andmeasured by a meter or other suitable means (not shown) or may be usedin any desired manner to exert a controlling effect.

The apparatus shown in FIG. 1 up to and including phase sensitivedetectors 31 and 32 is essentially similar to that described in mycopending application Ser. No. l46,269, filed May 24, I971 (transmittedto the Patent Office May 20, 1971) for Condition Measuring System andoperates in essentially the same manner and for the same purpose, exceptthat in the present arrangement separate phase detectors 31 and 32 areprovided for developing separate outputs respectively representative ofthe susceptive and conductive components of admittance exhibited by theprobe 20, whereas in the arrangement of the former application only anoutput representative of the susceptive component was developed.Briefly, in the present arrangement, oscillator 10, combiner 11, andbuffer amplifier 12 cooperate, as in the former arrangement, to providea highly stable, low impedance source of high frequency signal forsupply to the bridge circuit 17 whose voltage does not vary apreciablyeven though the conductance exhibited by probe 20 may become quitelarge. As in the former arrangement, the impedance presented bycapacitor 22, across which the bridge output is developed, is made muchlower than that exhibited by probe 20 so that there will be producedacross it quadrature components representative respectively of thesusceptive and resistive. components exhibited by the probe. Detector31, supplied with an in-phase component of the output from bufferamplifier 12, produces an output representative of the susceptivecomponent of the probe admittance, whereas detectpr 32, supplied with aninety degree out-of-phase component of the buffer amplifier output,produces an output representative of the conductive' component of theprobe admittance. As pointed out in my copending application aboveidentified, the impedance buffer 26 may comprise a highly stableamplifier having a high impedance input.

The impedance presented by capacitor 22 is preferably small and ofstable phase angle. This can be either a physical element or may be areflected or virtual impedance. For example it could be provided by atransformer primary connected in place of capacitor 22. In

this case if the secondary was unloaded, the primary inductive reactancewould constitute the impedance. If the transformer secondary wereheavily loaded,-the impedance reflected into the primary wouldconstitute the impedance. Similarly, if the input ofa high gainamplifier having a current feedback were connected in place of 22, thevirtual input impedance of the amplifier with feedback would constitutethe impedance.

For a better understanding of the operation of the remainder of thearrangement of FIG. 1 reference will be the susceptance curve, and thatif it is multiplied by an appropriate constant (P) and the resultingcurve P X conductance) is subtracted from the susceptance curve, therewill be obtained a curve of susceptance minus P times conductance whichis substantially independent of variations in moisture content. Inparticular, for'a variation in moisture from 0 to 1 percent, thedifference curve varies less than 1 percent. Accordingly,

by taking the difference between susceptance and P times conductance, aquantity can be obtained which is representative of dry' weight of theparticles with an accuracy'of better than 1 percent.

This may be accomplished in the arrangement of FIG. 1. The output offilter 36 corresponds to the signal represented by the susceptance curveof FIG. 2. With appropriate gain settings (scale factors) of the R.F.amplifier 30, the potentiometer 41 in the variable voltage divider canbe calibrated in P", such that the output of the variable voltagedivider corresponds to the signal represented by the P times conductancecurve of FIG. 2. Both the output signals represented by the susceptanceand P times conductance curves of FIG. 2 are supplied directly to theinput of differential amplifier 42, the output of which will correspondto the susceptance minus P times conductance curve of FIG. 2 and will berepresentative of dryweight of the polyester granules in which the probeis immersed. Switch 43 therefore is positioned upward to supply theoutput of differential amplifier 42 directly to the input of outputamplifier 44, and switch 47, which may be ganged to switch 43 as shown,is positioned downward to disconnect the output of divider 45 from theinput of amplifier 44.

While, as is evident from the foregoing, the arrangement describedprovides a substantial improvement in accuracy of measurement of dryweight under the particular circumstances shown in FIG. 2, it ispossible to obtain similar improvement even under less favorablecircumstances as will be explained later.

Referring now to FIG. 3 curves 50 and 51 represent respectively plots ofsusceptance and conductance versus percent moisture for corn. It will beobserved that the two curves, unlike those of FIG. 2, are substantiallydifferent in form and therefore it is not feasible to subtract themdirectly to obtain a curve which is substantially independent ofvariations in moisture over a wide range. This is because the water isactually absorbed into the corn and is not stratified as with thepolyester granules. In accordance with the invention it still ispossible to derive a quantity which is substantially independent ofvariations in moisture content. To do this, the form of the conductancecurve is modified by putting a capacitor in series with the probe. Theaddition of the series capacitor will lower both the susceptance and theconductance curves as illustrated by curves 52 and 53 in FIG. 3. Theeffect will be greater on the conductance curve since the change inconductance is greater. It is possible, therefore, to select the correctvalue of the series capacitor which will pull the condcutance curve downso that it will follow the susceptance curve with minimum error, as doesthe curve 54 designated 1.93 conductance in FIG. 2. Further it is notedthat the correct value of the series capacitor to be used will depend onthe amount of material present and being measured. As the level of thematerial changes, the value of the series capacitor needed for minimumerror should also change. This objective can be achieved by using aninsulated probe e.g. one coated with teflon. Subtraction of curve 54from curve 52 obtained in this manner yields the dotted curve 55exhibiting very small variation with moisture content, and thereforeproviding an accurate indication of dry weight of the com. This resultis obtained using the arrangement of FIG. 1 in the same manner asdescribed with reference to FIG. 2 except that an insulated probe isused and a capacitor of suitable magnitude is included in series withthe probe.

There are two factors which affect the shape of the susceptance andconductance curves as seen by the instrument. First is the frequency ofoscillation and second, the series capacity of the probe or thethickness of the insulation of the probe. Both factors may be adjustedto give the best fit between the conductance and susceptance curves andthereby minimize the error in the output. If for certain materials thismethod does not provide the accuracy needed, the arrangement of FIG. 1Amay be used.

FIG. 1A shows a modification of the system of FIG. 1 for achieving thisresult, the arrangement of FIG. 1A being substituted for the portion ofthe system of FIG. 1 beginning with filters 36 and 37. The outputs fromfilters 36 and 37, representing respectively the susceptive andconductive components, are supplied to the input of a divider circuit109 which produces at its output a signal representative of the ratio ofthe conductive and susceptive components. This in turn is supplied toinput of a function generator 110 for developing a signal proportionalto the ratio of conductance and susceptance, which is supplied tomultiplier 111 along with the suscept ance component from the output offilter 36'. The resultant product is subtracted from the susceptancecomponent in differential amplifier 112 to yield an output which isindependent of moisture and representative of dry weight. This output isamplified in output amplifier 113 and may be used in any desired manner.

The manner in which this result is achieved will be seen fromconsideration of FIG. 4, where there are shown the same curves 50 and 51of susceptance and conductance versus moisture content for corn as thoseof FIG. 3. Theincrease in the susceptance is a function of both theamount of material present and the percentage moisture. The ratio ofconductance to susceptance on the other hand is a function of thepercent moisture only. Thus if the appropriate function of theconductance to susceptance ratio times the susceptance is subtractedfrom the susceptance itself the result will be curve 52 which can bemade independent of the precent moisture and dependent directly upon thedry weight of material provided the appropriate function is used in thefunction generator shown in the circuit given in FIG. 1A. Theappropriate function for a given material can be derived. The outputsignal of the instrument is to be independent of moisture; consequentlyfrom the curves of FIG. 4 it is seen that the desired output signal isequal to the output representative of the susceptance of the materialwhere moisture content is zero (So).

Output S Sf(G/S) So Therefore f (6/5) 1 So/S For a given material thefunction can be found experimentally by determining the susceptance andconductance for a given sample of material for various amounts ofmoisture content. A curve representative of this function for corn isshown at 53 in FIG. 4, whose ordinates are numerical values according tothe vertical scale at the right-hand side of the graph. Curve 54 in FIG.4 shows the result of multiplying these ratios by susceptance fordifferent values of per cent moisture.

Similar results, but with somewhat less precision, can be obtained usingthe arrangement of FIG. 1 in the following manner. In FIG. 1, if switch43 is in its down position and switch 47 in its closed position, theoutput of the variable voltage divider 41 which is representative of theweight of water will be divided by the output of the differentialamplifier 42 which represents the dry weight. The output of the divider45 and the output of the output amplifier 44 will be representative offraction of the water relative to the dry weight or, with appropriatescale factors, the percent of water to dry weight. If the water contentis to be expressed as a percentage of total weight, the output ofvoltage divider 41 may be divided by the sum of the output ofdifferential amplifier 42 and the output of voltage divider 41 individer circuit 45 by appropriate changes in the inputs to circuit 45,as will be apparent.

In the arrangement of FIG. 1A, the function generator 110 may be of anysuitable conventional form. For example it may be of the form shown inthe Burr-Brown Research Corporation Handbook and Catalog of OperationalAmplifiers", LI-227, page 48, lower half, Copyright I969. In thisfunction generator, the operational amplifier A may be, for example, ofthe form shown in RCA Data Sheet File No. 360, issue dated Noember,I968. Differential amplifier 112 may be of the form shown in the sameBurr-Brown catalog above referred to, on page 41, identified as SimpleCircuit. Again in this circuit the operational amplifier A may be of theform shown 'in the RCA Data Sheet above identified. Divider 109 may beof the form shown in The Microelectronics Data Book", Second Edition,December, 1969, Motorola Semiconductor Products, Inc., Section onMultipliers, Modulators and Detectors, Linear Four Quadrant Multiplier,MCI595L, FIG. 13.

Referring now to FIG. 5, there is shown a modification of the embodimentof FIG. 1 in which subtraction of the conductive and susceptivecomponents of the bridge output is accomplished using a singlephasesensitive detector phased to detect at a specific angle selected tohave a value between and 90 as will be explained persently. Thecircuitry of FIG. 5 is substituted for all of that following theimpedance buffer 26 in FIG. 1. It comprises an R.F. amplifier suppliedwith the output from impedance buffer 26 of FIG. 1. The output of RF.amplifier 80 is supplied to the input of phase sensitive detector 81which also is supplied with a phase-shifted signal through phase shifter82 whose input is supplied in phase with the output of buffer amplifier12 of FIG. 1. The operation of phase detector 81 is explained withreference to FIG. 5A in which the susceptive and conductive componentsof the bridge output signal are represented by the vectors B and Grespectively and the detector phase angle is a. In operation thedetector will produce a dc. output proportional to the difference of themagnitudes of B and G. By appropriate selection of the angle a betweenthe detecting phase and the conductive component G, the relativecontribution of G can be made such that the d-c output of the detectoris proportional to |B| P|G|, where P is the appropriate constant and theoutput is representative of dry weight of the material measured. Thus,in FIG. 5A, it is observed that B 8 sin a and G G cos a where a is thaangle between the detecting phase and the conductive component G. Thedetected signal is B' G but Therefore the detected signal isproportional to B I- P |G I where P cot a and sin a is theproportionality constant.

Similarly, as will be seen later, an angle of 45 can be used for themeasurement of FIG. 8 and 8A and the output will be representative ofthe true liquid level. As in the system of FIG. 1, the output of thedetector may be supplied through filter 83 to an output d-c amplifier84, the output of which may be used as desired.

Another representative application of the invention is in the coating ofbox board with a thin coating of clay to render it suitable forreceiving printing inks and like materials. In this process clay in theform of a slurry is applied to box board in a continuous procedure toform a coating of only a few thousandths of an inch in thickness verymuch thinner than the box board itself which may be as much as 40 timesthicker than the clay coating. It is essential that the coating be asthin and uniform as possible to provide the desired surface forreceiving ink, but thick enough to provide complete coverage and fillall depressions in the board surface. To this end it is desired tocontinuously monitor the thickness of the coating immediately followingits processing and to control the coating machinery accordingly. Themeans used to monitor the coating thickness must do so without directlycontacting the coated surface which is still moist and sticky. Priorknownmeasurement methods, such as those involving nuclear techniques,are unsuitable for this purpose because they are subject to error due tovariation in the thickness of the board stock itself. In accordance withthe present invention means are provided for continuously monitoring theconductance of the coating between two spaced points as it passescontinuously from the coating machinery without contacting the coatedsurface.

To accomplish this, the arrangement shown in FIG. 6 may be substitutedfor the straight capacitive probe 20 in the system of FIG. 1. In thisarrangement the box board 90, with its caly coating 91, is shown movingin the direction indicated by the arrow 92 as it leaves the coatingmachinery (not shown). Capacitive electrodes 93 and 94 contact theuncoated side of the board at spaced points along its direction oftravel and are both connected through coaxial cable 21 to point 1% ofthe bridge circuit in the arrangement of FIG. 1.

The equivalent circuits of this arrangement are shown in FIGS. 6A and6B, in which R represents the resistance of the coating between the twopoints at which the electrodes are located and X represents the totalseries capacitive reactance of both electrode 93 and 94. Thus the totalimpedance 2 exhibited by the electrodes is R jX and the admittance maybe expressed as Y= G +jS. This is the admittance presented to the bridgecircuit at point 19b in FIG. 1, whose conductive and susceptivecomponents are represented by resistor 18 and capacitor 19 in thatfigure. As demonstrated below, the conductance G of the coating betweenelectrodes 93 and 94 can be expressed in terms of the conductance G andsusceptance S as:

G=G (1 +K )=G= ]S(l +K )/K] G= S/K From (1 and (2):

G G (l K) v G [l (S/G'F] c s /G To obtain the quantity G in accordancewith the expression. the arrangement of H6. 1 is further modified bysubstituting the arrangement shown in FIG. 7 for that portion of theFIG. 1 arrangement beginning with an to the right of filters 36 and 37.The output of filter 36. representing the d'susceptive component S inthe expressio,is supplied to a squaring circuit 100 to produce an outputrepresentative of S which is supplied to one input of a divider circuit101. The output of filter 37, representing the conductive component G issupplied to the other inputof divider 101. The output of divider 101,representative of SIG', is supplied to one input of an adder circuit102, the other input of which is supplied with the output (G') fromfilter 37.

The output from adder 102 will then represent G S /G' G, the conductanceof the coating on the box board measured between the capacitiveelectrodes 93 and 94 in FIG. 6. This may be amplified as before in anoutput amplifier 103, the output of which may be used to control thecoating process.

In the arrangement of FIG. 7, the squaring circuit may be of the formshown in the same urr-Brown catalog hereinbefore identified, page 48,upper left. Divider 101 may be of the same form as divider 45 of theFIG. 1 arrangement previously identified.

Another representative application of the invention rlates to themeasurement of level of a condcuting material'by an immersion probewhere the conducting material tends to produce a coating on the probe toa level higher than the actual level of the liquid. Referring to FIG. 8it is observed that the coating 106 acts like an infinite series stringof small resistors between the top of the probe coating and the top ofthe liquid level, the probe insulation 105 acts like an infinite numberof small shunting capacitors. Such a circuit can be represented as atransmission line, where the series impedance per unit length (Z) isdependent upon the coating and the probe configuration, and the shuntadmittance per unit length (y) is dependent upon probe insulation andthe probe'configuration. Since the level measurement is made at radiofrequency, a short length of coating (several inches) on the probe willact like an infinitely ling transmission line. The impedance of aninfinite transmission line is its characteristic impedance (Z which isgiven by:

The series impedance per unit length, z, is merely the where f is thefrequency at which the measurement is made and C is the capacitance perunit length of the probe. Therefore:

This impedance due to the coating as seen by the instrument can berepresented by a parallel circuit as shown in FIG. 8A where themagnitude of R is equal.

to the magnitude of X,:

|R.|=|x.| (pm 10 The impedance as seen by the instrument due to thematerial level on the probe is the capacitive reactance of the length ofprobe (1) which is immersed in the material:

Using the present invention two d.c. signals are developed, oneproprtional to probe-to-ground conductance and one proportional toprobe-to-ground susceptance. By subtracting one from the other an outputwhich is proportional only to the actual material level is obtained.This is shown as follows:

Instrument output alBl lGl l/IX I l/|Xc|- 1/ IRCI But |X,| |R Therefore:Output a ll] X l Consequently, for a coating which is long enough to berepresented by an infinite transmission line (in most cases a maximim ofseveral inches), the acoting is ignored and the actual material level ismeasured.

An alternate method of obtaining the same result is to phase thedetectot at 45 so that the subtraction is performed at detection andonly one detector is required. A vector representation of the detectorscheme is shown in FIG. 58. Taking the vector sum of the htree signalsdetected, it is seen that the output depends only on the level and isindependent of coatings built up on the probe.

While the invention has been described with reference to certainpreferred embodiments. it will be apparent that many changes may be madeand many widely different embodiments may be constructed withoutdeparting from the scope of the invention, which is defined by thefollowing claims:

What is claimed is:

1/ In a system for measuring a first characteristic of a material, anadmittance probe adapted for association with said material, means forderiving first and second signals respectively representative only ofthe susceptive and the conductive components of admittance exhibited bysaid probe, said susceptive component varying with said firstcharacteristic and also varying with a second characteristic of saidmaterial, said conductive component varying only with said secondcharacteristic of said material, and means for combining said first andsecond signals to yeild a signal primarily representative of variationsin said first characteristic and substantially independent of variationsin said second characteristic.

2.A system according to claim 1 in which said admittance probe isincluded in an admittance measuring circuit supplied from a source ofhigh frequency energy, said source comprising a highly stable oscillatorand a high gain amplifier provided with high feedback so as to provide avery low impedance output.

3. In a system for measuring the dry weight of a aterial having avariable moisture content, an admittance probe adapted for associationwith said material, means for deriving first and second signalsrespectively representative only of the susceptive and the conductivecomponents of admittance exhibited by said rpobe, said susceptivecomponent varying with the weight of said material and also varying withthe moisture content thereof, said conductive component varying onlywith the moisture content of said material, and means for combining saidfirst and second signals to yield a signal primarily representative ofvariations in weight of said material and substantially independent ofvariations with moisture content.

4. In a system for measuring the conductivity between spaced points acoating applied to a relatively non-conductivesubstrate, a pair ofspaced electrodes positioned in proximity to said substrate on the sidethereof opposite said coating, means for deriving first and secondsignals respectively representative only of the susceptive andconductive components of admittance exhibited by said electrodes, andmeans for operating on said first and second signals to produce aquantity representative of the conductivity of said coating between saidspaced electrodes.

5. A system according to claim 4 in which said last means comprises:means for squaring said first signal, means for dividing said squaredsignal by said second signal, ane means for adding the signal resultingfrom said division to said second signal to provide a signalrepresentative of said conductivity.

6. A system according to claim 1 in which said combining means comprisesmeans for subtracting said second signal from said first signal.

7. A system according to claim 1 including means for modifying at leastone of said signals prior to combination with the other of said signalsin a manner to render the variation of both said signals with saidsecond characteristic of substantially the same form.

8. A system according to claim 7 in which said signal modiyfing meanscomprises a capacitor in series with said probe.

9. A system according to claim 7 in which said signal modification iseffected by using an insualted probe.

10. A system according to claim 7 including means for dividing saidsecond signal by said first signal, a function generator for producing asignal which is a predetermined function of the quotient signal producedby said division, means for multiplying said lastnamed signal by saidfirst signal, and-means for subtracting the resultant product signalfrom said first signal to produce a signal primarily representative ofvariations in said first characteristic and substantially independent ofvariations in said second characteristic.

11. A system according to claim 10 in which the signal produced by saidfunction generator is such that, when multiplied by said first signal,the resultant product signal is similar in mathematica form to saidfirst signal.

12. In a system for measuring a first characteristic of a material,means including an admittance probe adapted for association with saidmaterial for deriving a signal having components respectivelyrepresentative only of the susceptive and conductive components ofadmittance exhibited by said probe, said susceptive component varyingwith said first characteristic and also varying with a secondcharacteristic of said material, said conductive component varying onlywith said second characteristic of said material, and a phasesensitivedetector supplied with said signal and phased to produce an outputrepresentative of the difference between said susceptive and conductivecomponents.

13. In a system for measuring a first characteristic of a material,means including a capacitive probe adapted for association with saidmaterial for deriving a signal having components respectivelyrepresentative only of the susceptive and conductive components ofadmittance exhibited by said probe, said susceptice component varyingwith said first characteristic and also varying with a secondcharacteristic of said material, said conductve component varying onlywith said second characteristic of said material, and a phase-sensitivedetector supplied with said signal and phased to produce an outputrepresentative of the difference between said susceptive component andthe product of a constant times the conductive component.

herence of said material to said probe when the level of said materialchanges, said conductive component varying only by reason of saidadherence of material to said probe, and a phase snesitive detectorsupplied with said signal and phased to produce an output representativeof the difference between said susceptive and conductive components.

' l i 'l =l

3. In a system for measuring the dry weight of a aterial having avariable moisture content, an admittance probe adapted for associationwith said material, means for deriving first and second signalsrespectively representative only of the susceptive and the conductivecomponents of admittance exhibited by said rpobe, said susceptivecomponent varying with the weight of said material and also varying withthe moisture content thereof, said conductive component varying onlywith the moisture content of said material, and means for combining saidfirst and second signals to yield a signal primarily representative ofvariations in weight of said material and substantially independent ofvariations with moisture content.
 4. In a system for measuring theconductivity between spaced points o a coating applied to a relativelynon-conductive substrate, a pair of spaced electrodes positioned inproximity to said substrate on the side thereof opposite said coating,means for deriving first and second signals respectively representativeonly of the susceptive and conductive components of admittance exhibitedby said electrodes, and means for operating on said first and secondsignals to produce a quantity representative of the conductivity of saidcoating between said spaced electrodes.
 5. A system according to claim 4in which said last means comprises: means for squaring said firstsignal, means for dividing said squared signal by said second signal,ane means for adding the signal resulting from said division to saidsecond signal to provide a signal representative of said conductivity.6. A system according to claim 1 in which said combining means comprisesmeans for subtracting said second signal from said first signal.
 7. Asystem according to claim 1 including means for modifying at least oneof said signals prior to combination with the other of said signals in amanner to render the variation of both said signals with said secondcharacteristic of substantially the same form.
 8. A system according toclaim 7 in which said signal modiyfing means comprises a capacitor inseries with said probe.
 9. A system according to claim 7 in which saidsignal modification is effected by using an insualted probe.
 10. Asystem according to claim 7 including means for dividing said secondsignal by said first signal, a function generator for producing a signalwhich is a predetermined function of the quotient signal produced bysaid division, means for multiplying said last-named signal by saidfirst signal, and means for subtracting the resultant product signalfrom said first signal to produce a signal primarily representative ofvariations in said first characteristic and substantially independent ofvariations in said second characteristic.
 11. A system according toclaim 10 in which the signal produced by said function generator is suchthat, when multiplied by said first signal, the resultant product signalis similar in mathematica form to said first signal.
 12. In a system formeasuring a first characteristic of a material, means including anadmittance probe adapted for association with said material for derivinga signal having components respectively representative only of thesusceptive and conductive components of admittance exhibited by saidprobe, said susceptive component varying with said first characteristicand also varying with a second characteristic of said material, saidconductive component varying only with said second characteristic ofsaid material, and a phase-sensitive detector supplied with said signaland phased to produce an output representative of the difference betweensaid susceptive and conductive components.
 13. In a system for measuringa first characteristic of a material, means including a capacitive probeadapted for association with said material for deriving a signal havingcomponents respectively representative only of the susceptive andconductive components of admittance exhibited by said probe, saidsusceptice component varying with said first characteristic and alsovarying with a second characteristic of said material, said conductvecomponent varying only with said second characteristic of said material,and a phase-sensitive detector supplied with said signal and phased toproduce an output representative of the difference between saidsusceptive component and the product of a constant times the conductivecomponent.
 14. In a system for measuring the level of a conductingmaterial in a vessel, an admittance probe adapted for immersion intosaid material, said material tending to produce a coating on aid probe,means for deriving a signal having components represnetative only of thesusceptive and conductive components of admittance exhibited by aidprobe, said susceptive component varying with said level and alsovarying by reason of adherence of said material to said probe when thelevel of said material changes, said conductive component varying onlyby reason of said adherence of material to said probe, and a phasesnesitive detector supplied with said signal and phased to produce anoutput representative of the difference between said susceptive andconductive components.